Field of the Invention
The invention relates to a process for introducing zinc into a water-containing component of the primary system of a boiling-water reactor or pressurized-water reactor nuclear power plant, in particular for establishing a zinc level to prevent the deposition of radioactive cobalt and/or to reduce the possible susceptibility of materials in the component to corrosion.
Such a water-containing component of a nuclear power plant can be the primary circuit of a boiling-water reactor or pressurized-water reactor plant.
Consequently, the feed may be delivered into a pipe which is linked into the primary part or is part of the primary part.
The water in the component may contain radioactive elements, such as radioactive cobalt. With time, these radioactive elements are incorporated in oxide layers which form on the internal surfaces of such a component. This results in sources of radioactive radiation in the oxide layers, entailing expensive protective measures when repair and maintenance work needs to be carried out on the component.
It is known from European Patent EP 0 257 465 B1 that the incorporation of radioactive cobalt into the oxide layers can be prevented if the element zinc is introduced into the water situated in the component. This is because zinc occupies positions in the oxide layers which would otherwise be taken up by the radioactive cobalt. The controlled introduction of zinc into the oxide layers considerably reduces the incorporation of radioactive cobalt.
Consequently, the sources of radioactive radiation in the oxide layers are advantageously reduced.
Moreover, zinc improves the protective properties of oxide layers on component surfaces, leading to a reduction in the possible susceptibility of materials forming the component to corrosion.
European Patent EP 0 257 465 B1 discusses the introduction of zinc in the form of zinc salt. However, it is established that the anions which enter the water, which is located in the component, together with the zinc salt damage the internal surface of the component. This is because the anions can have a corrosive effect on the internal surfaces of the component. Therefore, it is proposed in European Patent EP 0 257 465 B1 to introduce zinc oxide instead of a zinc salt into the water which is located in the component.
However, the solubility of zinc oxide in water is extremely low.
Consequently, feeding of zinc oxide is a complicated operation. It is necessary to form a slurry or to introduce the zinc oxide in portions, requiring a solid metering system. This is technically complex and susceptible to faults, owing to the tendency of the metering systems to become blocked.
Another possibility is for sintered zinc oxide pellets to flow through with a partial stream of the reactor feed water, but this option requires an expensive pressure-resistant and temperature-resistant passive metering installation.
Zinc can also be used to prevent corrosion. U.S. Pat. No. 3,580,934 has disclosed that corrosion of a water pipe can be prevented by forming a protective layer of silicate which contains zinc on the internal surface of the pipe. A zinc salt, but in particular a complex of a zinc salt with sulfamic acid, is used to form this protective layer. It is not possible to employ this technique in the nuclear part of a nuclear power plant, since it is not permitted to introduce a silicate into this area.